1. Field of the Invention
The present invention is in the field of video compression and more particularly data reduction in a video image data reduction system, wherein both static and dynamic video images can be stored using less data to adequately define the image.
2. Description of the Related Art
Data reduction is particularly useful in a video telephone application where there is a very demanding requirement for data reduction so that the transmission falls within the bandwidth or bit rate capacity of the telephone line. Additionally, there is a requirement in multimedia applications to be able to store static and dynamic video images efficiently. Such data may be stored on CD-ROM or transmitted over computer networks. Video image data reduction has a long history, stemming largely from the need to reduce the bandwidth required to transmit the video signal in a video telephone service, since a high bandwidth leads to high system costs. Typically, data reduction schemes have been directed to sacrificing resolution and/or motion rendition in a user acceptable way. For example, in the early 1970's, the Picturephone.TM. developed by Bell System required a bit rate of a few megabits per second. The essential data reduction step utilized in Picturephone was to store and process the image at the sender terminal by effecting a comparison between sequential images or frames and then transmit only that information defining picture elements that changed from frame to frame. This approach, known as "frame differencing," still forms an essential step in the most recent art in this field.
Generally then, it is known to compress video images by employing the following three step process:
Step 1. Data Acquisition
This provides for the digital video image to be represented as a two-dimensional array of pixels, with the two dimensions specifying the horizontal and vertical dimensions of the image. Each pixel comprises a data item describing the color properties of the pixel. When more bits are used, more colors or gray levels are represented by the image.
Step 2. Data reduction
The data reduction step is the heart of any video compression. The purpose of this step is to eliminate the picture information that is irrelevant to the human eye. A video signal can be described as an amplitude function where amplitude can be interpreted as brightness or color. Typical video compression methods currently employed transform this function into the frequency domain, typically using discrete cosine transforms. A threshold then eliminates frequencies that the human eye is insensitive to. These transformations are computationally intensive and therefore require specialized hardware which makes them expensive. For moving images, there is a further data reduction step comprising the comparison of successive frames of video so that only the differences are processed.
Step 3. Data compression.
This step compresses the reduced data from step 2 using well known data compression methods such as Huffman encoding or run length encoding.